1. Field
The invention relates to a process for the preparation of a masterbatch in the liquid phase of one or more diene elastomers, in particular a natural rubber latex, and of one or more fillers.
2. Description of Related Art
The term “masterbatch” is understood to mean an elastomer-based composite into which one or more fillers and optionally other additives has/have been introduced.
It is known that, in order to obtain the optimum reinforcing properties conferred by a filler in a tire tread and thus a high wear resistance, it is advisable generally for this filler to be present in the elastomeric matrix in a final form which is both as finely divided as possible and as homogeneously distributed as possible. In point of fact, such conditions can only be achieved in so far as this filler exhibits a very good ability, on the one hand, to be incorporated in the matrix during the mixing with the elastomer and to deagglomerate and, on the other hand, to disperse homogeneously in this matrix.
In point of fact, in order to improve the dispersibility of the filler in the elastomeric matrix, it is known to resort to a mixing of elastomer and filler in the “liquid” phase. In order to do this, recourse is had to an elastomer in the latex form and to an aqueous dispersion of the filler, commonly referred to as “slurry”.
The process for the preparation of the masterbatch in the liquid phase comprises a coagulation stage which is generally initiated by the addition of coagulating agent to the medium. U.S. Pat. No. 5,763,388 teaches a process for the preparation in the liquid phase of a masterbatch of a polymer latex and of silica as filler, comprising the incorporation within the latex of a modified silica. This silica has reacted beforehand with a coupling agent, this modification to the silica making it possible to uniformly disperse the modified silica within the polymer latex. According to this document, the coagulation stage is carried out in the presence of a coagulating agent.
The filler can also be carbon black. In this field, the problem of the uniform dispersion of the fillers and in particular carbon black within the rubber had already been posed from 1955. Thus, there is known, from the document BE 541816, a process for the preparation of a masterbatch of rubber and of carbon black in the liquid phase. This process is carried out continuously and uses hydraulic impacts or intense mechanical stirring to bring about the dispersion of the carbon black within the elastomeric matrix.
More recently, the document WO97/36724 discloses a process for the preparation of a masterbatch and a specific apparatus which makes it possible to improve the dispersibility of the carbon black in a natural rubber latex. This technology meets two objectives: the coagulation stage is carried out in the absence of coagulating agent and a masterbatch having a uniform distribution of the filler is obtained. However, this technology exhibits a number of disadvantages. The apparatus employed is extremely complex and the process described is based on very precise characteristics related to this apparatus, such as a defined geometry of the coagulation zone or also a defined difference in flow rate.